The present invention relates to a document or image reading device for use in a facsimile system or other optical equipment. More particularly, the invention relates to improvements in a large-scale image sensor that has recently been developed as a replacement for an MOS photodiode array or CCD image sensor currently used in document or image reading devices.
A large-scale image sensor of the general type to which the invention pertains is composed of an insulating substrate, a plurality of photoelectric transducers formed on the substrate, and a circuit for switching and scanning the transducers which is either formed on the substrate or attached to an adjacent substrate, with the length of the transducer array being equal to the size of the document or image. The sensor uses an optical fiber array or lens array to read the original employing an image formation ratio of 1:1. With this arrangement, the length of optical path needed for image formation can be decreased to significantly reduce the size of the reading device.
The construction of the conventional large-scale image sensor is shown in FIGS. 1A-1C. In FIG. 1A, reference numeral 1 indicates a photoelectric transducer formed of a thin photo-conductive film, the equivalent circuit of which is a photodiode PD and a capacitor CD. Reference numeral 5 designates an array of MOS transistors for switching the transducer, 6 a shift register for scanning the MOS transistor array, 8 a signal line, 9 a load resistor, and 10 a bias supply.
FIG. 1B is a plan view showing a specific integrated circuit implementation of the transducer of FIG. 1A, and FIG. 1C is a cross-sectional view of FIG. 1B taken along a line A--A'. The photoelectric transducer 1 is constructed with a substrate 11 supporting a thin photo-conductive film 3 sandwiched between discrete electrodes 2 made of a thin conductive film and a continuous electrode 4 made of a thin transparent conductive film. A switching circuit 7, in the form of an IC chip which includes the MOS transistor 5 and the shift register 6 fabricated on a semiconductor substrate, is mounted on the substrate 11 and connected to the photoelectric transducer by wire bounding or other suitable means.
When the image of the original is formed on the transducer 1 by an optical system (not shown), a photo-current that varies with the luminous intensity is produced on the various photodiodes PD. The resulting change in the potential of the electrode 2 is transmitted over the signal line 8 by sequentially closing the corresponding MOS transistors 5 to thus develop an output video signal across the resistor 9.
FIG. 2 is a schematic diagram of the sensor for one picture element. The n-channel MOS transistor generally indicated at 5 is formed on a p-type silicon substrate 12 and includes n-impurity diffused areas 13 and 14, a gate 15, a drain 16, and a source 17. The drain 16 is connected to a discrete electrode on the photoelectric transducer 1 via the thin amorphous silicon film 3. The transparent electrode 4 is negatively biased with a bias supply 10. The source 17 is grounded through a signal line 8 and a load resistor 9. The substrate 12 is grounded (as is usually the case when an n-channel MOS transistor is operated).
The operation of the circuit of FIG. 2 will now be described with reference to the equivalent circuit thereof shown in FIG. 3. A constant current source 19 corresponds to the photoelectric transducer 1 of FIG. 2. A capacitance 20 and a diode 21 are formed at the interface between the substrate 12 and the impurity diffused area 13. While the n-channel MOS transistor 5 is off, a photo-current Ip flows from the current source 19 upon illumination of the film 3 with light, whereupon a signal charge accumulates on the capacitance 20. After sufficient time has passed for the charge to accumulate the gate of the MOS transistor 5 is pulsed (by the shift register 6) to turn on the transistor 5 and to discharge the signal charge through the signal line 8 and resistor 9. Signal detection is effected by sensing the resulting discharging current.
In actual operation, as the signal charge accumulates on the capacitance 20, the potential at point A decreases (becoming negative), whereupon the diode 21 becomes biased in the forward direction. If the resulting voltage is larger than the forward voltage drop across the diode 21, the diode conducts. As a result, the accumulated signal charge is discharged through the diode. Because the potential at point A cannot decrease beyond the forward voltage drop of the diode, only a small discharging current can flow when the MOS transistor is turned on. This results in insufficient signal detection. Consequently, only a small output signal is produced upon illumination, which output signal is not linear with respect to luminous intensity and is easily saturated. A large-scale image sensor having these characteristics has a low sensitivity and low dynamic range, and is far from being efficacious in many practical applications.